We are developing an intrument for the measurement of the 2-photon absoption cross-sections, of biological releint fluorepohores over a wide wavelenght range. The excitation source is a broadly tunable (700nm - 1 000nm) modelocked Ti-sapphire laser. Excitation into the visible range is possible using a modelocked dye laser. To avoid intensity errors due to pre-dispersed of the ultra short laser pulses in the instruments optics, the laser pulses are predispessed to a duration of about 1 pico second. By moldulating the excitation light we are also able to distinguish between two and three photon excitation effects.